PROJECT 2 ABSTRACT Perfluoroalkyl substances (PFASs) are widely used synthetic chemicals present in nonstick coatings, food packaging, and water repellant clothing. Perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS), perfluorononanoic acid (PFNA), and perfluorohexane sulfonate (PFHxS) are the four primary PFASs found in human blood samples in United States. There is accumulating evidence to suggest that PFAS exposures during the sensitive windows of pregnancy and gestation may adversely affect hormones involved in metabolism and adipogenesis. Specifically, exposure to PFASs during pregnancy/in utero may be related to excessive pregnancy-related weight gain, reduced duration of breastfeeding following pregnancy, and increased risk of overweight or obesity during childhood. These are important outcomes because e xcessive weight gain during pregnancy confers lifelong risk of morbidity for mothers and infants alike; a shortened duration of breastfeeding reduces the associated health benefits for both mother and child, and overweight/obesity during childhood increases long term risk of diabetes, hypertension, and other serious health disorders for affected children. Thus,identifying modifiable risk factors for these conditions has become a public health priority. By taking advantage of data and existing biological samples from women and children enrolled in the New Hampshire Birth Cohort Study, we will determine whether women with higher exposures to PFAS during pregnancy/gestation have greater risks for: 1) excessive weight gain during pregnancy, 2) shorter duration of breastfeeding or altered breast milk composition, and 3) offspring with greater childhood adiposity through age 5. We will also explore whether pregnancy weight gain or duration of breastfeeding are mediators of the association between prenatal PFAS exposures and adiposity at age 5. The United States for PFOA and PFOS. levated levels of these substances have been detected in drinking water supplies throughout the United States including in Northern New England PFAS exposures are preventable. Our study will provide critical data to inform policy reform and Environmental Protection Agency recently established drinking water health advisories E . identify possible opportunities to reduce or prevent PFAS exposure and thus improve the lifelong health of women and children.